The treatment of water is currently of constantly increasing importance. In addition to drinking water, especially in the chemical and pharmaceutical industries, high-purity process waters are required which must be prepared in a large quantity as inexpensively as possible. High-purity water, in addition, is especially also required in the semiconductor industry, for example, for rinsing silicon wafers, in particular after etching processes. The purity requirements of the water are known to be particularly high in this sector.
Known processes that provide ultrapure water are multistage processes comprising a first stage in which the raw water is softened and/or already partially desalinated, a second stage in which the water from the first stage is further purified in a pressure-driven membrane separation process, and a third stage in which the water is finally substantially completely deionized, for example, by electrodeionization (EDI). Additional process steps, for example, to eliminate organic impurities, can be provided.
Water softening and/or desalination in the first stage generally proceed by use of one or more ion exchangers. For the softening, cation exchangers in the sodium form are preferably used, whereas for the desalination, combinations of cation and anion exchangers are customary. The total ionic load of the water to be treated can be markedly reduced already by such methods.
Membrane separation processes which come into consideration are, in particular, reverse osmosis and nanofiltration, optionally also in combination with one another. If relatively large amounts of dissolved carbon dioxide are present in the raw water, an additional degassing step can be provided before or after the membrane separation process.
If a high water yield is of importance, the concentrate from the membrane separation stage can be treated in a further additional membrane separation stage.
Electrodeionization appliances, in customary designs, always require a solution which takes up the ions separated off from the water to be treated and discharges them (concentrate) from the device. That solution flows through at least one concentrate chamber, and the water that is to be treated through at least one diluate chamber. A high ionic conductivity in the concentrate chambers is known to be achieved, in particular, by the following:                an addition, e.g., of neutral salts, is conducted,        the concentrate being recirculated through the concentrate chambers such that the ions that are separated off accumulate there or        the concentrate chambers (as also optionally the diluate chambers) are packed with ion-exchange resins.        
WO 2010/054782 A1 discloses a multistage process of treating water in which a water stream is fed to a first membrane separation device where the water stream is divided into a concentrate stream and a permeate stream. The permeate stream is fed to a downstream electrodeionization appliance, the concentrate stream is processed in a second membrane separation device. The permeate exiting from the second membrane separation device is fed into the concentrate chambers of the downstream electrodeionization appliance and further utilized in this manner. Concentrate exiting from the electrodeionization appliance can optionally be fed back into the inlet of the first membrane separation device.
It could therefore be helpful to improve known water-purifying processes having a sequence of membrane separation devices and electrodeionization appliances, in particular with respect to the water yield to be achieved.